The application is the first-filed application for this invention.
The invention relates to a method and an apparatus for attenuating stray current flowing through ground return circuits in the vicinity of an animal susceptible to be affected by the current.
When a stray current impulse from the concrete floor of a livestock barn flows through the body of an animal to reach the metallic structures of that building and its grounding connection network, the animal is affected by this stray current impulse. This usually causes a physiological reaction of the animal resulting into abnormal health conditions.
For example, the physiological reaction of a human being when an electric current pulse flows through his body (hand-trunk-foot) can be summarized as follows: slight perception threshold: 1 to 8 mA rms; painful sensation threshold: 9 to 80 mA rms; dangerous threshold: greater than 81 mA rms, for a duration of 1 second.
Stray current will cause symptoms specific to each species. For example, cows will refuse to be milked and to enter the barn and may kick the milker. In the most critical situations, the cows will present problems of mastitis, of reproduction, of somatic cell count, etc.
Pigs can present similar symptoms: cases of cannibalism and mastitis, and problems of diarrhea, of temperature and of constipation may be found. In the most critical cases, the death rate of the piglets may considerably increase.
A stray voltage is a potential difference existing between two points susceptible to be contacted simultaneously by an animal to cause a flow of current through the animal""s body. A stray voltage can also be defined as a potential difference between two points caused by a flow of current through the body of an animal.
A stray voltage can produce a flow of current both through the body of the animal from the concrete floor of the livestock barn toward metallic structures of that livestock barn contacted by the animal (touch voltage circuit), and through the animal""s body from one paw to the other (step voltage circuit).
For a same path through a body, the danger for the animal depends essentially on the intensity and the duration of the travel of the current. The most serious criteria is the admissible value of the contact voltage, that is the product of the current passing through the body and its impedance, as a function of time. The relationship between the current and the voltage is not linear since the impedance of the body depends on the frequency of the signal.
Different portions of the body, such as the skin, the blood, the muscles and other tissues and articulations, have a certain resistive and capacitive impedance.
The internal body impedance (Zi) can be considered to be mainly resistive. However, studies demonstrate that a low capacitive element is also present.
The skin impedance (Zp) can be considered to comprise a set of resistive and capacitive elements. Its structure is composed of a semi-conducting layer and small conducting elements (the pores). The skin impedance decreases rapidly as the current increases.
The value of the skin impedance varies depending on the voltage, the frequency, the duration of the passage of the current, the surface of contact, the pressure of the contact, the humidity factor of the skin and the temperature.
The total body impedance (Zt) is composed of a resistive and a capacitive element.
Since the skin impedance varies according to the frequency, the total body impedance is greater with a DC current and decreases as the frequency increases.
The skin impedance decreases as the frequency increases.
At the moment when the contact voltage is applied, the body capacitive elements are not charged, that is why the skin impedances Zp are negligible and the initial body resistance (Ri) is approximately equal to the internal body impedance Zi. The resistance Ri limits the short current peaks.
The industry has employed several different hardware solutions to reduce overvoltages. According to a simple reactor solution, three inductors are provided, a separate one of the inductors placed in series with each of the three supply lines between an Variable Speed Drive (VSD) and the three motor terminals.
According to another solution, a sine wave filter is linked to the supply lines wherein this filter includes three capacitors and three inductors. A separate inductor is positioned in series with each supply line. One capacitor is linked between each pair of supply lines.
According to yet another solution, a dv/dt filter is linked to the three supply lines between an VSD and a motor. The filter includes three inductors, three resistors and three capacitors. Again, a separate inductor is positioned in series with each supply line. A separate resistor is linked in series with a separate capacitor between each pair of supply lines.
According to one other solution, a resistor-inductor-diode (RLD) filter is linked to the supply lines. The RLD filter includes six diodes, three inductors and two resistors. A separate inductor is positioned in series with each supply line. The diodes are arranged in series pairs to form three parallel diode legs between positive and negative terminals. A node between the diodes of each leg is linked to a separate supply line and the positive and negative terminals are connected through separate resistors to positive and negative DC drive buses, respectively.
While each of the overvoltage solutions identified above effectively reduces overvoltages, each solution suffers from at least one and typically a plurality of the following shortcomings. Some of the shortcomings of these prior art systems are: they are configured using relatively large components and therefore require large volumes, they require a large number of components and therefore are relatively expensive to configure, they provide only poor/slow dynamic response to a motor load, they create periodic instability, they cause line-to-line neutral voltage to be undamped, they cause resonant conditions in line-to-neutral voltage, they cause rise times which vary as a function of cable length, and/or they can only be used with specific cable lengths.
An object of the present invention is therefore to provide a method and system for efficiently neutralizing stray voltage and current impulses that can flow through an animal""s body which is kept in a building having a metallic structure.
Another object of the invention is to provide a method and system for attenuating stray voltage and current impulses in a ground return circuit, by adding an inductive component which, when coupled to the parasitic capacitance of the charge, creates a reactive filter effect.
According to a first broad aspect of the present invention, there is provided a method for attenuating a stray current impulse flowing through a ground return circuit in a proximity of an animal susceptible to be affected by the stray current impulse. The method comprises providing an impedance having one coil for each power line at an input of an electrically conductive element; connecting the impedance in series between the output of a power source and the electrically conductive element, the electrically conductive element being connected to the ground return circuit; whereby attenuation of the stray current impulse limits capacitive leakage which affects the animal and whereby an inductance of the impedance, coupled to a parasitic capacitance of the electrically conductive element, creates a reactive filter effect.
Preferably, the coils are magnetically coupled via a common ferrite core. Preferably, the coils have a same number of windings. Preferably, a summation of ampere turns of the coils is equal to zero;
More specifically, in accordance with the present invention, there is provided a method for neutralizing a stray voltage produced in a ground return circuit and a stray current flowing through the ground return circuit in the proximity of an animal susceptible to be affected by the stray voltage and current impulses.
In this method for neutralizing a stray current impulse, there are two preferred embodiments: in the capacitive mode (motor), the subtransmission of the voltage (dv/dt) can be modified in the winding of the motor to decrease the frequency; in the resistive mode, a series impedance can be inserted in the circuitry (fencer).
The method for attenuating the stray current impulse limits the capacitive leakage by modifying the response in frequency and attenuates the current value and the circulation of the impulse on the ground return network (Z2).
The method for attenuating the stray current impulse comprises introducing an inductance value which, when coupled to the charge parasitic capacitance, creates a reactive filter effect. Since the effect is of the reactive type, the impulse energy is not absorbed. However, this solution allows to start with a situation wherein short and intense impulses are transformed into a permanent regime at low amplitude.
The apparatus comprises a core in which are included 1, 2, 3 or 4 coils. The apparatus is typically a high quality reactance which includes a low reactive value with respect to the line current (residual mode) and a high reactive value with respect to the parallel current (common mode).